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Application of the Quartz Crystal Microbalance to the Evaporation of Colloidal Suspension Droplets N. T. Pham,†,§ G. McHale,† M. I. Newton,*,† B. J. Carroll,‡ and S. M. Rowan† School of Science, The Nottingham Trent University, Clifton Lane, Nottingham NG11 8NS, United Kingdom, and Unilever Research Laboratory, Quarry Road East, Bebington, Merseyside L63 3JW, United Kingdom Received September 12, 2003. In Final Form: December 3, 2003 An investigation into the evaporation of sessile droplets of latex and clay particle suspensions is presented in this work. The quartz crystal microbalance (QCM) has been used to study the interfacial phenomena during the drying process of these droplets. Characteristic changes of the crystal oscillating frequency and crystal resistance (damping of the oscillating energy) have been observed and related to the different stages of the evaporation process. Measurements have been made for latex particle sizes from 1.9 to 10 µm and for rough and polished crystals using drops from 0.3 to 1.5 µL. The behavior of the QCM is shown to depend strongly on the size of particles present and on the morphology of the crystal surface. One of the most striking features is a drastic damping of the oscillation energy and corresponding rise in frequency observed during the final stages of evaporation, particularly for the clay suspensions.
Introduction The evaporation of a liquid drop on a surface is a welldocumented1-3 subject and has also been recently investigated using the quartz crystal microbalance (QCM). Joyce et al. studied the evaporation of sessile drops of a homologous series of alcohols on the crystal surface.4 They found that the QCM is capable of monitoring the extreme modes of droplet evaporation and is a powerful tool for the analysis of sessile droplet evaporation. McKenna et al. studied the evaporation of small droplets of water using simultaneous QCM frequency measurements and videomicroscopy.5 They showed that QCM data could be interpreted in terms of an evaporation sequence involving pinned contact area and retreating contact line periods, that absolute frequency changes related to droplet position on the electrode, and that compressional wave resonances could occur for noncentrally located droplets. Separate from liquid droplet measurements, the QCM is known to be a very sensitive gravimetric method both for mass deposited from the vapor phase and for mass deposited from the liquid phase. The QCM has therefore been used widely to study interfacial phenomena including gas adsorption kinetics,6 adsorption/desorption in liquids,7,8 and wetting velocities of surfactants.9,10 Additionally, the †
The Nottingham Trent University. Unilever Research Laboratory. § Present address: School of Physics, University of Edinburgh, Kings Buildings, Mayfield Road, Edinburgh EH9 3JZ, U.K. E-mail:
[email protected]. ‡
(1) Fuchs, N. A. Evaporation of droplet growth in gaseous media; Pergamon Press: London, 1959. (2) Picknett, R. G.; Bexon, R. J. Colloid Interface Sci. 1977, 61, 336. (3) Bourges-Monnier, C.; Shanahan, M. E. R. Langmuir 1995, 11, 2820. (4) Joyce, M. J.; Todaro, P.; Penfold, R.; Port, S. N.; May, J. A. W.; Barnes, C.; Peyton, A. J. Langmuir 2000, 16, 4024. (5) McKenna, L.; Newton, M. I.; McHale, G.; Lucklum, R.; Schroeder, J. J. Appl. Phys. 2001, 89, 676 and references therein. (6) Hamilton, C.; Gedeon, A. J. Phys. (Paris) 1986, E19, 4271. (7) Weerawardena, A.; Drummond, C. J.; Caruso, F.; McCormick, M. Langmuir 1998, 14, 575. (8) Schumacher, R. Angew. Chem. 1990, 294, 329. (9) Lin, Z.; Hill, R. M.; Davis, H. T.; Ward, M. D. Langmuir 1994, 10, 4060. (10) Lin, Z.; Stoebe, T.; Hill, R. M.; Davis, H. T.; Ward, M. D. Langmuir 1996, 12, 345.
QCM is also used in monitoring thin films,11-13 the adhesion of polymers,14,15 and in biotechnology as well as to study the cell and protein adhesion kinetics.16-22 The drying of suspensions has vital effects for certain industries, such as washing, printing, and coating.23,24 For example, paint manufacturers use a variety of additives to ensure that the pigment is evenly dispersed and remains so during drying.25 Furthermore, small latex spheres are widely used as binders in coating to determine the mechanical properties of the latter.26 Despite these interests, there have been only a few studies into the behavior of these suspensions during evaporation. In one of the studies, it was found that a ringlike deposit can be observed whenever suspension drops containing dispersed solids evaporate on a surface.27 This can be explained by contact line pinning (clp) of the suspension drop during (11) Okahata, Y.; Ebara, Y. J. Chem. Soc., Chem. Commun. 1992, 116. (12) Okahata, Y.; Ariga, K. J. Chem. Soc., Chem. Commun. 1987, 1535. (13) Bailey, L. E.; Kambhamapatai, D.; Kanazawa, K. K.; Knoll, W.; Frank, C. W. Langmuir 2002, 18, 479. (14) Wang, J.; Ward, M. D.; Ebersole, R. C.; Foss, R. P. Anal. Chem. 1993, 65, 2553. (15) Tanaka, S.; Iwasaki, Y.; Ishihara, K.; Nakabayashi, N. Macromol. Rapid Commun. 1994, 15, 319. (16) Barnes, C.; D’Silva, C.; Jones, J. P.; Lewis, T. J. Sens. Actuators 1991, B3, 295. (17) Gole, A.; Sainkar, S. R.; Sastry, M. Chem. Mater. 2000, 12, 1234. (18) Zhou, T.; Marx, K. A.; Warren, M.; Schulze, H.; Braunhut, S. J. Biotechnol. Prog. 2000, 16, 268. (19) Satjapipat, M.; Sanedrin, R.; Zhou, F. Langmuir 2001, 17, 7637. (20) Chong, K. T.; Su, X.; Lee, E. J. D.; O’Shea, S. J. Langmuir 2002, 18, 9932. (21) Hook, F.; Rodahl, M.; Brzezinski, P.; Kasemo, B. Langmuir 1998, 14, 729. (22) Fredriksson, C.; Kihlman, S.; Rodahl, M.; Kasemo, B. Langmuir 1998, 14, 248. (23) Brinker, C. J.; Hurd, A. J.; Schunk, P. R.; Frye, G. C.; Ashley, C. S. J. Non-Cryst. Solids 1992, 147, 424. (24) Brinker, C. J.; Scherer, G. W. Sol-gel science: The physics and the chemistry of sol-gel processing; Academic Press: San Diego, CA, 1990; Chapter 1. (25) Deegan, R. D.; Bakajin, O.; Dupont, T. F.; Huber, G.; Nagel, S. R.; Witten, T. A. Phys. Rev. 2000, E62, 756. (26) El Bediwi, A. B.; Kulnis, W. J.; Luo, Y.; Woodland, D.; Unertl, W. N. Mater. Res. Soc. Symp. Proc. 1995, 372, 277. (27) Deegan, R. D.; Bakajin, O.; Dupont, T. F.; Huber, G.; Nagel, S. R.; Witten, T. A. Nature 1997, 389, 827.
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evaporation. In fact, Deegan et al. found that clp and evaporation are sufficient conditions for ring stain formation.28 To ensure that the contact line of a drop is pinned during at least a significant part of the evaporation, there must be a continuous radial outward flow of material from the center of the contact area to the contact line of the drop to compensate for the liquid removed by evaporation. This flow and the particulate mass it carries are responsible for the building up of the ring stain, which is observed after complete evaporation. Contact line pinning was also observed by Parisse and Allain29 who reported that the radius of the contact base between the colloidal suspension drop and the solid substrate remains constant for a considerable part of the drying process. Contact line pinning is very sensitive to the surface roughness and the chemical heterogeneity of the surface. However, the drying of colloidal suspensions has not been reported with the QCM system. Although there has been an investigation into the electrostatic adsorption of polystyrene nanospheres from a solution onto the modified quartz crystal surface, the measurements were taken ex situ after complete drying and do not reflect the evaporation process.30 In this paper, we extend an earlier work31 in which we reported the experimental results for the ring formation of 1.9 µm diameter latex particles on polyester fiber surfaces used as optical light guides and the light attenuation this caused. In this report, the investigation has been extended to larger latex and clay particles and, furthermore, the quartz crystal microbalance has been used to characterize the different stages in drying of latex and clay particle suspension droplets on the crystal surface. The results are interpreted using existing QCM and evaporation models. If a uniform thin rigid (solid) mass layer is deposited onto the crystal surface, then the relationship between the frequency shift ∆f and the additional mass ∆m is given by the Sauerbrey equation:32
∆f )
-2f02 ∆m xF µ A
( )
(1)
q q
where f0 is the crystal resonance frequency, A is the piezoelectrical active area, and Fq and µq are the density (2648 kg m-3) and the shear modulus (2.947 × 1010 kg m-1 s-2) for AT-cut quartz, respectively. The frequency change is simply due to the increased effective thickness of the crystal and is only valid for very thin mass layers. In this case, there is no damping of the crystal energy due to the deposited mass layer. If the quartz crystal is immersed in a liquid or a liquid drop is present on its surface, the simple rigid mass model no longer applies. Kanazawa and Gordon33 treated the liquid as a purely viscous fluid where the shear wave displacement induced in the liquid by oscillation of the crystal surface was described as a damped cosine function, which decays to 1/e of its original amplitude at a decay length or penetration depth δ of the shear wave,
δ)
x
ηL πf0FL
(2)
where ηL is the liquid viscosity and FL is the liquid density; (28) Deegan, R. D. Phys. Rev. 2000, E61, 475. (29) Parisse, F.; Allain, C. J. Phys. II 1996, 6, 1111. (30) Serizawa, T.; Takeshita, H.; Akashi, M. Langmuir 1998, 14, 4088. (31) Pham, N. T.; McHale, G.; Newton, M. I.; Carroll, B. J.; Rowan, S. M. Langmuir 2002, 18, 4979.
a typical penetration depth of a shear wave in water for a 5 MHz crystal in water is around 0.25 µm. The Kanazawa and Gordon model predicts a frequency shift dependent on the square root of the density-viscosity product of the liquid to which the crystal is exposed and is given by
x
FLηL πFqµq
∆f ) -f03/2
(3)
The crystals do not show uniform sensitivity across their surface but have a reduced sensitivity toward the outer edges; this is particularly important when looking at droplets on the surface.5 In addition to the frequency shift, the entrainment of liquid at the crystal-liquid interface causes energy dissipation,
x
D ) 2f01/2
FLηL πFqµq
(4)
For deposition of a thin uniform layer of mass onto a crystal from a liquid phase, the overall QCM response is a Sauerbrey frequency shift with no further dissipation beyond the original dissipation due to the liquid. In this paper, we report the evaporation of colloidal particle suspension droplets on the surface of a quartz crystal. Initial work investigates the reproducibility of the QCM system by evaporation of small drops of distilled water and investigates the effect of the surface roughness of the individual crystal used; the inclusion of roughness is an extension to the work in ref 5. Droplets of monodisperse spherical latex particle suspension, which act as model systems, with a range of particle sizes from 1.9 to 10 µm are then introduced to investigate the effects of the additional solid phase during drying. The conclusions drawn from these systems are then applied to a system of polydisperse clay particle suspensions. The results are interpreted using the QCM theory given above and the behavior of drying of suspensions known from optical observations. Experimental Section An AT-cut quartz crystal of 25 mm diameter and of resonance frequency of 5 MHz with gold electrodes was used. To take the crystal surface properties into account, both polished and unpolished crystals were used. Figure 1 shows scanning tunneling microscopy (STM) images of the surfaces of these crystals. The crystals were mounted in a holder in which the contacts for the electrical signals were integrated. The crystal holder was connected to a PLO-10 (Maxtek Inc., U.S.) phase lock oscillator that provided the crystal frequency and an output voltage proportional to the crystal resistance; this is a measure of the energy dissipation factor D.34,35 These were measured using an Agilent frequency counter and a Keithley 175A digital voltmeter connected to a microcomputer using a GPIB interface. Charge-stabilized particle suspensions of fluorescent monodisperse spherical polystyrene latex particles of 1.9, 3.15, 5, and 10 µm diameter (Duke Scientific) were used. They were obtained as aqueous suspensions with 1% w/w of the total solution. Lower concentration suspensions were prepared by diluting with distilled water. These particles have a refractive index of 1.59 at 589 nm and a density of 1.05 g cm-3. Clay particle suspensions were also investigated. The clay was montmorillonite (Gelwhite GP). The dispersion was prepared in 0.2% NaCl, which is known (32) Sauerbrey, G. Z. Phys. 1959, 155, 206. (33) Kanazawa, K. K.; Gordon, J. G., II Anal. Chim. Acta 1985, 175, 99. (34) Lee, S.-W.; Hinsberg, W. D.; Kanazawa, K. K. Anal. Chem. 2002, 74, 125. (35) PLO-10 datasheet: http://www.maxtekinc.com/products/research/ plo10.htm.
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Figure 2. The evaporation of a droplet of distilled water, with a volume of 0.5 µL, from (a) an unpolished and (b) a polished crystal. The crystal resistance axis corresponds to the upper curve, and the frequency axis to the lower curve. Figure 1. STM images of gold electrodes, showing the surface roughness of polished (a) and unpolished (b) gold electrodes on quartz crystal. to facilitate the delamination process,36 the process being carried out in an ultrasonic bath. The dispersion obtained had a roughly Gaussian size distribution with a modal diameter of 900 nm and range of 100-5000 nm. It had a natural pH of about 9.5, although the initial pH of the mother NaCl solution was about 5-6. The refractive index of the montmorillonite particles is from 1.55 to 1.57, dependent on crystal orientation, and the density of the solid is about 2.5 g cm-3. The particles are of a nonuniform, platelike shape, which has been described as similar to pieces of torn-up paper.36 The QCM system was left to stabilize before a droplet of particle suspension of known volume and solid concentration was deposited onto the center of the quartz crystal; the use of small drops deposited centrally minimizes the effect of the Gaussian radial sensitivity distribution and also avoids compressional wave resonances.5 The drops were left to evaporate in an isolated area of laboratory at ambient conditions; under these conditions, similar drop volumes took the same evaporation time to within (1.5%. The suspension was well mixed before extraction, and a microsyringe was used for depositing the drops. The figures shown in the following results section are typical examples of crystal response extracted from a much larger set of experiments.
Results and Discussion (a) Evaporation of Distilled Water on Unpolished and Polished Crystals. A typical change in frequency f and crystal resistance R of the QCM during the evaporation of a 0.5 µL drop of distilled water on an unpolished crystal is shown in Figure 2a; values are plotted as changes ∆f and ∆R from the values prior to deposition. The graphs show three distinct regions. An instantaneous decrease in f and increase in R are observed when the water drop is deposited, followed by a plateau for a long period of time and finally a sudden recovery of the values (36) Lagaly, G.; Ziesmer, S. Adv. Colloid Interface Sci. 2003, 105, 100.
for the frequency and resistance to those prior to the deposition of the water drop. This pattern can be explained as follows: f initially decreases on deposition due to an area of the crystal surface being wetted by the drop where previously it was exposed to air. The values of the initial changes depend on the viscosity-density product of the water forming the droplet; R increases due to the additional damping caused by the liquid. Due to the clp of the water drop during a significant period of the evaporation, both frequency and resistance signals remain more or less constant; the duration of clp is enhanced by the surface roughness of the crystal. During this period, the evaporation occurs mainly by reduction of the drop height and not its contact area with the crystal. Since the typical value for δ in water is around 0.25 µm, the crystal does not therefore sense the change in the drop height during this stage. In the final stages of evaporation, the contact line unpins and rapidly retreats, reducing the drop contact area as well as the drop height and causing a sharp change in f and R; the final values are the original values prior to deposition because once evaporation has completed no residue is left on the crystal. The response on a polished crystal, Figure 2b, for the same evaporation process shows some minor differences. The long period of clp where both f and R are essentially constant is reduced compared to the unpolished case. After the deposition of the water droplet, the evaporation starts off in a manner similar to that of the unpolished crystal with reducing droplet height (decreasing contact angle) and pinned contact area; this is responsible for the constant signals of f and R. This process lasts until a critical value for the contact angle is reached as determined by the contact angle hysteresis between the advancing and the receding values, after which the contact area starts to decrease. The two signals f and R change steadily until in the final stages of the evaporation a sudden recovery of f and R to the original, pre-droplet-deposition values occurs. The polished crystal shows that complete clp occurs
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Figure 3. The evaporation of a droplet of 0.5 µL of 1.9 µm monodisperse latex particle suspension with a solid concentration of 1% w/w on (a) an unpolished and (b) a polished crystal. The crystal resistance axis corresponds to the upper curve, and the frequency axis to the lower curve.
during a much shorter duration, and this is consistent with earlier work.5 (b) Evaporation of a Polystyrene Latex Particle Suspension. A typical pattern of the crystal characteristics during the evaporation of a 0.5 µL drop of 1.9 µm latex particle suspension of 1% w/w solid concentration is given in Figure 3. Both unpolished and polished crystals show a similar behavior. f decreases and R increases when the suspension drop is deposited onto the crystal. This is followed by a steady increase of R which then rises sharply before finally dropping back to a stable value somewhere close its original value before the deposition of the droplet. Following droplet deposition, the frequency f remains constant to a first approximation and then decreases suddenly before finally returning to a stable value clearly distinguishable from its value prior to the deposition of the suspension. This final value of frequency is lower than that for the crystal prior to droplet deposition. The response of the polished and unpolished crystals differs primarily in the period preceding the sharp rise in R and fall in f. The responses from the unpolished and polished crystals can be interpreted using eqs 1-4 describing Sauerbreylike mass deposition and Kanazwa and Gordon type viscosity-density liquid response. Similar to the case of pure water, the deposition of the suspension drop causes f to decrease and R to increase due to the replacement of part of the crystal-air interface with a crystal-liquid interface. This causes a viscosity-density product dependent change in the frequency and an additional damping of the shear wave energy. The frequency f then remains constant which marks the stage of clp and reducing the droplet height; the lack of change in frequency indicates no significant deposition of particles occurs from the droplet suspension. This is followed by a period where the particles move within the liquid out toward the edges of the drop as a result of the evaporation. During this
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period, more latex particles approach the crystal surface and cause an increase in the density-viscosity product within δ, thus resulting in a higher damping of the crystal oscillation which is seen in the slow increase of R. The densities of the latex particles (1.05 g cm-3) and water (1.0 g cm-3) are very close, so that to a first approximation only the viscosity within δ of the shear wave changes due to particle deposition. It is probable that the particles are not rigidly coupled at this stage to the crystal surface since water is still present which will act as a layer between the particles and the crystal surface, a so-called water bridge or capillary bridge. In the final stages of the drying process, these bridges would then be removed by evaporation. The surface tension (capillary forces of these water bridges) of the evaporating water will start to pull the particles closer to the surface of the crystal and so would cause higher damping of the crystal oscillation, which then accounts for the sharp rise in the R signal (indicated by the arrow). This hypothetical, surface-tension-induced coalescence of the particle with the surface is also reflected by a reduction in the magnitude of the f signal as seen in Figure 3a. Once all the liquid has evaporated, R recovers to a value comparable with that obtained prior to the deposition of the drop of the suspension, implying that a rigidly bound, Sauerbrey-like mass layer is present on the crystal. An estimate of the amount of the additional mass can be made from the net change in frequency before and after deposition. Figure 4 shows the relationship between deposited latex mass and the frequency shift; the masses were determined using the solid concentration and the volume of the suspension drop. Given that the piezoelectrically active area is a disk of radius r ) 0.25 cm, eq 1 gives a proportionality constant of 288 Hz µg-1 compared to the 285 Hz µg-1 obtained from the graph. The experimentally determined value is in good agreement with the theoretical value, suggesting that the 1.9 µm particle layer can be treated as a rigid mass layer deposition even though it is clearly not a uniform layer of mass. However, the deposited mass is in the form of a ring stain and not a uniform film so the radial sensitivity of the QCM makes eq 1 an overestimation. This overestimate is not believed to be significant because the radius of the deposited mass layer is only around 1 mm at its maximum and care was taken to deposit the suspension drop at the center of the crystal. Therefore, to a first approximation the radial sensitivity can be neglected. The linearity of the solid line in Figure 4 supports this assumption, since if the radial sensitivity is affecting the crystal response one would expect a relative reduction in ∆f values for larger drops. Figure 4 also shows the relationship between the volume of the drop and ∆f; these data were taken 1 min after the deposition of the drops for the sample to reach equilibrium state. Lin and Ward showed a linear relationship between ∆f and the square of contact radius r of the liquid drop with the crystal in the case of assuming that the radial sensitivity function is relatively flat at the crystal’s center, so that the Gaussian sensitivity behavior can be disregarded.37 By simple geometry, the contact radius can be related to the suspension drop volume (V) and finally one obtains the relationship ∆f ∝ V2/3,37 which can be seen in Figure 4 (broken line). The interpretation of the crystal responses for the evaporation of 1.9 µm latex particle suspension droplets of the same solid concentration on a polished crystal (Figure 3b) is very similar to that on an unpolished crystal. However, one can see clearly a period of linear decrease (37) Lin, Z.; Ward, M. D. Anal. Chem. 1996, 68, 1285.
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Figure 4. (a) The frequency shifts obtained after drop evaporation as a function of the volume of suspension drops (diamonds and left ∆f-axis) and the overall mass of latex particles (squares and right ∆f-axis). (b) The frequency shifts obtained after drop evaporation as a function of the volume of clay suspension drops (diamonds and left ∆f-axis) and the overall mass, clay and NaCl (squares and right ∆f-axis) (clay suspension of 5 g L-1 concentration and 0.2% NaCl).
in f and increase in R, at a time of approximately 15 min, just before the sharp rise in both signals, which is not observed on an unpolished surface. Similar to the case with pure water, the polished crystal surface does not give rise to complete clp and the water contact line starts to recede once a critical value of the contact angle is reached. This slow receding of the contact line explains the part of the response exhibiting a linear change in f and R just before the sharp rise, as seen in Figure 3b. The evaporation of 0.5 µL of latex suspensions with larger particle diameters of 3.15, 5, and 10 µm with 1% w/w solid on an unpolished quartz crystal is shown in Figure 5. The crystal characteristics in Figure 5a show that there is little qualitative difference in QCM response between the 1.9 and 3.15 µm particles. Further, the values for the change in frequency ∆f before and immediately after the droplet deposition are comparable in all three cases (∆f ≈ -130 Hz). This suggests that the number and the sizes of the particles in the suspension do not significantly vary the contact angle or wetted area of the drop on the crystal surface. This fact has also been confirmed by videomicroscopy. However, larger particles, such as 5 and 10 µm, show some differences. Not only is the overall value for f larger for increasing particle size, the shape of the R graph for the final stage of evaporation (at around 11 min in Figure 5c) seems also to depend on the particle diameter. Using a Sauerbrey interpretation and the overall ∆f for the predeposition to postevaporation
Figure 5. The evaporation of 0.5 µL of monodisperse latex particle suspension of (a) 3.15 µm, (b) 5 µm, and (c) 10 µm particle diameter with 1% w/w solid concentration on unpolished crystal. The crystal resistance axis corresponds to the upper curve, and the frequency axis to the lower curve.
frequency change shows that the deposited mass increases with the particle diameter. However, the solid concentration was the same for all three particle sizes and hence only the number of particles varied while the mass was constant. Looking at the R response, one can clearly see that even after complete drying there is a considerable damping in the crystal oscillation energy (especially for the largest size of 10 µm). The loss in energy occurring when the larger latex particles have been left as a residue on the QCM indicates that the Sauerbrey-like rigid mass view cannot be used to interpret the frequency data. The overall values for R further suggest either that the dried particle layer possesses some viscoelasticity, which increases with increasing diameter so resulting in the higher R values for larger particles, or that an alternative energy loss mechanism is occurring. It is plausible that the degree of contact between the crystal surface and the particles may determine the viscoelastic behavior of the particles. Figure 1 shows that the unpolished crystal is scattered with craters whose widths are around 2-3 µm. Larger particles therefore are only loosely bound to the surface, as some particles might sit on the edge of those craters, and therefore cause damping of the crystal energy. The 1.9
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Figure 7. The evaporation of 0.5 µL of polydisperse clay particle suspension droplets with 0.75% w/w clay concentration and 0.2% NaCl on unpolished (a) and polished (b) crystal. The crystal resistance axis corresponds to the upper curve, and the frequency axis to the lower curve.
Figure 6. The evaporation of 0.5 µL of monodisperse latex particle suspension of (a) 3.15 µm, (b) 5 µm, and (c) 10 µm particle diameter with 1% w/w solid concentration on polished crystal. The crystal resistance axis corresponds to the upper curve, and the frequency axis to the lower curve.
and 3.15 µm particles can fill in such craters and will therefore be more strongly bound to the surface. Hence, for the unpolished crystal surface the effect of 1.9 and 3.15 µm particles may conform more to a rigid mass interpretation while the larger particles show some nonrigid mass behavior. However, one of the most striking features in Figure 5c is the rise in frequency above the initial predeposition value (indicated by the arrow) and the associated dissipation. Frequency rises above the value for a crystal in air are unusual but have been recorded during evaporation of drops of distilled water when those drops were noncentral and the drop heights fulfilled resonant conditions for compressional (i.e., sound) waves.5 Recent work by Laschitsch and Johannsmann38 has also demonstrated a frequency rise and associated damping when a small sphere is brought into contact with a QCM in air; they attributed the frequency rise to added stiffness in the system and the generation of sound waves. The results for the evaporation of 0.5 µL of latex suspension with particle diameters of 3.15, 5, and 10 µm with 1% w/w solid on a polished quartz crystal are given in Figure 6. These are very similar to the ones obtained (38) Laschitsch, A.; Johannsmann, D. J. Appl. Phys. 1999, 85, 3759.
from an unpolished crystal. However, the overall R values for particle diameters of 5 and 10 µm are smaller than those obtained in the unpolished crystal case. (c) Evaporation of a Polydisperse Clay Particle Suspension. The deposition of 1.9 µm monodisperse latex particles suggests that the Sauerbrey rigid mass model may be used to study the deposition of clay. The clay particle suspension is a more complex system. First, it is polydisperse with regard to size, and second, the particle shape is flat and lamellar rather than spherical. The particles also tend to have a more complex surface charge distribution, which itself is pH-dependent. At low pH, the flat faces of the particles are anionic but the edges are cationic, arising from the protonation of aluminol or silanol groups.36 Above about pH 5, however, this region also becomes negatively charged, so that at the working pH (9.5) in the present systems, the particles carry a negative charge all over, although this charge may still not be uniformly distributed. The presence of electrolyte in the system plays an important role in these experiments. As evaporation proceeds, the ionic strength increases, from an initial value of about 40 mM through to saturation in the final stages. This change in the ionic strength will affect the interactions between the particles themselves (the electrokinetic and thus the surface potentials fall with increasing ionic strength)39 and also the interaction of the particles with the crystal surface. The latter interaction may in turn affect the particles’ propensity to pin the three phase contact line as the evaporation process proceeds. Typical results for evaporation experiments of clay particle suspensions on an unpolished and a polished crystal are shown in Figure 7. The responses of both crystals are similar to the ones obtained with the 1.9 µm latex particles with a nonzero net change ∆f and a vanishing net change in dissipation after complete drying. This implies that the dried “submicron” clay particles on the crystal surfaces (39) Chorum, M; Rengasamy, P. Eur. J. Soil Sci. 1995, 446, 657.
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can be considered as a rigid mass layer deposition. To confirm this is the case, Figure 4b examines whether the relationship between the deposited mass ∆m and the frequency shift ∆f is linear. In this case, the experimentally obtained proportionality constant of 213 Hz µg-1 (solid line) is lower than the theoretical value. This might be due to the nonuniformity of the shape of the clay particles and hence a random distribution of orientation of the platelike particles on the crystal surface. It was shown in an earlier work that the QCM response for clay particles deposited “edgewise” differed from those deposited “facewise”.40 Furthermore, the presence of the second solid phase (NaCl) will also have an effect on the crystal response. Finally, the latex deposits can largely be described by a single layer of particles, which will not necessarily be the same for the clay deposits. Note that, unlike in the case for deposition of the latex particles, we were unable to use videomicroscopy to identify individual clay particles. A second layer shearing on the underneath layer in contact with the crystal surface may well have an effect on the crystal response. Furthermore, a deviation from the relationship of ∆f ∝ V2/3 for the clay particle suspension drop is observed in Figure 4b (broken line). Nevertheless, the theoretical and the experimental proportionality constants are in the same order of magnitude (40) Shirtcliffe, N. Colloids Surf., A 1999, 155, 277.
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and one can conclude that the clay particle deposition does resemble that expected for a rigid mass layer. Conclusion An investigation into the kinetics of the evaporation of pure water and aqueous particle suspensions using the QCM is reported. A well-established procedure for the evaporation of droplets of pure water on polished and unpolished crystal surfaces was used to characterize the QCM response and to interpret the more complex systems of monodisperse and polydisperse particle suspensions. A range of particle sizes have been used, and QCM results show strong size-dependent responses. While the rigid mass interpretation can be used to characterize deposits of the smaller particles, the larger ones must include a contribution to the response from dissipative effects. More interestingly, in some cases the results for the evaporation of aqueous colloidal suspensions suggest that the QCM reacts very sensitively to changes in the final stages of drying and may be a method to investigate capillary bridges between particles. Acknowledgment. The authors gratefully acknowledge John Yorke, Unilever Port Sunlight, for supplying the Gelwhite GP and the information about its properties. LA0357007
